Combining Unit
This was designed by Tommy Flowersof the Post Office Research Station at Dollis Hill in North London. It used thermionic valves (vacuum tubes) to implement the logic. This invoved the Boolean "exclusive or" (XOR) function in combining the various bit-streams. In the following "truth table", 1 represents "true" and 0 represents "false". (At Bletchley Park these were known as x and • respectively.)
| INPUT | OUTPUT | |
| A | B | A ⊕ B |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
Other names for this function are: "not equal" (NEQ), "modulo 2 addition" (without carry) and "modulo 2 subtraction" (without 'borrow'). Note that modulo 2 addition and subtraction are identical. Some descriptions of Tunny decryption refer to addition and some to differencing, i.e. subtraction, but they mean the same thing.
The combining unit implemented the logic of Tutte's statistical method. This required that the paper tape containing the ciphertext was tried against a tape that contained the component of the Lorenz cipher machine generated by the relevant two chi wheels at all possible starting positions. A count was then made of the total number of 0s generated, with a high count indicating a greater probability of the starting position of the chi key sequence being correct.
Read more about this topic: Heath Robinson (codebreaking Machine)
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